The present invention relates to a Y/C (luminance/chrominance) separation apparatus for separating a luminance signal and a chrominance signal from a composite color television signal.
According to recent digital processing techniques, a Y/C separation apparatus for separating a luminance signal and a chrominance signal from a composite color television signal is used.
For example, an adaptive type Y/C separation apparatus selecting a plurality of filters according to correlation between a reference signal and a signal near the reference signal in the horizontal or vertical direction as described in Japanese Patent Laid-Open 7-298291.
A Y/C separation apparatus in accordance with the prior art is explained below, referring to the drawing.
FIG. 6 is an example of a block diagram of Y/C separation apparatus in accordance with the prior art.
In FIG. 6, the blocks 601 and 602 are a first delay device and a second delay device delaying each input signal (composite signal) by one horizontal scanning period (lH, hereafter), respectively. The block 603 is a first chrominance signal bandpass filter (first BPF, hereafter) for extracting a chrominance signal from the output of delay device 601. The block 604 is a first comb filter for extracting a chrominance signal from the input signal and the output of delay device 601. The block 605 is a second comb filter for extracting a chrominance signal from the output signals of delay devices 601 and 602. The block 606 is a first correlation detection circuit for detecting a correlation between the input signal and the output signal of delay device 601. The block 607 is a second correlation detection circuit for detecting a correlation between the output signals of delay devices 601 and 602. The block 608 is a mixer for mixing the outputs of the filters 603, 604 and 605 according to the correlation results of correlation detection circuits 606 and 607. The block 609 is a second chrominance signal bandpass filter (second BPF, hereafter) for limiting a bandwidth of the output of mixer 608. The block 610 is a subtracter for outputting a luminance signal by subtracting a chrominance signal which is an output of second BPF 609 from the input signal delayed by 1H.
In the prior art described here, the signal is processed taking an input signal delayed by 1H as a center. Here, let us call the input signal delayed by 1H a reference signal.
The function of the Y/C separation apparatus composed like the above is explained below. The inputted composite signal is delayed at delay devices 601 and 602. The output of delay device 601 is defined as a reference signal. Comb filters 604 and 605 are composed by making differences between the reference signal and the signals apart from the reference signal by 1H, ahead and behind. The comb filters separate chrominance signal components by subtracting the signals apart from the reference signal by 1H from the reference signal.
A chrominance signal component of the reference signal is extracted by making the reference signal pass through first BPF 603. Correlation detection circuits 606 and 607 detect correlations (called vertical correlation) between the reference signal and the signals apart from the reference signal by 1H, ahead and behind (geometrically locates up and down the reference signal position)
Correlation is detected, for example, by filtering the absolute value of the difference between chrominance signal components of two signals and comparing the filtered output with a designated value. Mixer 608 selects a chrominance signal from the outputs of comb filters 604 and 605 and first BPF 603 according to the outputs of correlation detection circuits 606 and 607. That is, mixer 608 selects an output of comb filter 604 when a correlation between the reference signal and the signal ahead by 1H (one horizontal scanning period) exists, selects an output of comb filter 605 when a correlation between the reference signal and the signal behind by 1H exists and selects an output of first BPF 603 when neither correlation ahead nor behind exists. Thus, a signal passed an optimum filter is outputted according to the correlation between the reference signal and the signals ahead and behind by 1H.
Further, a chrominance signal can be outputted from second BPF 609 having a bandwidth for a chrominance signal by filtering the output of mixer 608. A luminance signal can be outputted from subtracter 610 by subtracting the above-described chrominance signal from the signal time compensated by delaying from the input signal by 1H.
However, according to the above composition, in the case in which a comb filter is adapted by misdetecting at correlation detection, for example, when the reference signal is only a luminance signal without chrominance component, a dot interference occurs and it is conspicuous.